(1) Field of the Invention
This invention relates to a crane provided with means for damping vertical vibrations for the safety of the crane, and more particularly to an overhead travelling crane provided with means for coping with vertical earth tremors of high intensity in the event of an earthquake.
(2) Description of the Prior Art
An overhead travelling crane of the prior art will be described by referring to FIGS. 1 and 2 showing the crane in its entirety and FIGS. 3 and 4 showing its travelling device in a fragmentary view.
As shown in FIGS. 1 and 2, an overhead travelling crane of the prior art comprises two parallel girders 1, connecting girders 12 each connecting the girders 1 together at opposite ends thereof, a travelling device 14 disposed on the undersurface of the opposite ends of the girders 1 for movement on rails 8a each supported on one of a plurality of saddles 38 formed on a shed 8, a pair of transverse rails 13B supported on the girders 1, and a trolley 13 provided with wheels 13A for movement on the rails 13B. As shown in FIGS. 3 and 4, the travelling device 14 comprises a pair of saddles 2 formed on the undersurface of each of opposite end portions of each girder 1, a pair of trucks 4 pivotally supported through a truck pin 3 by the pair of saddles 2, and a plurality of pairs of wheels 7 each pair being rotatably connected to one of the trucks 4 forming the pair in such a manner that the wheels 7 forming a pair are mounted at the front end and the rear end respectively of each truck 4 through a bearing 9 and an axle 10, so that the wheels 7 run on the rails 8A.
In the overhead crane of the prior art constructed as aforesaid, all the elements from the wheels 7 to the girders 1 are rigidly connected together, so that it is only the structural damping of the girders 1 that can be expected to have effects on vertical vibrations. Thus, the damping ratio of the crane is only 1-2%. Consequently, if the number of vibrations of high spectral density coincides with the natural frequency of vertical bending vibrations of the girders 1 in the event of an earthquake, the crane will be vigorously vibrated in response with the exciting force of the earthquake, thereby resulting in rupture of the girders 1 or breakage of wires when a load is supported by the girders.
In order to avoid resonance of the girders in the event of an earthquake, it is necessary to reduce the bending stress of the girders by increasing the natural frequency of the vertical bending vibrations of the girders with the notion that the frequency components of the earthquake are wide in range. However, if this requirement is met, it will be inevitable that the crane is increased in weight and the construction is uneconomical.